This invention relates generally to cylinder heads for closing the outer or head end parts of cylinders of internal-combustion engines and forming combustion chambers. More particularly, the invention relates to a cylinder head of a composite construction comprising a bottom wall part to face the combustion chamber of the cylinder and a back-up or reinforcement part on the side of the bottom wall part opposite to the combustion chamber.
A typical cylinder head of conventional design, as will be described more fully hereinafter, is an integral structure ordinarily it the form of a casting of aluminum, cast iron, or some other suitable metal. It is a complicated structure comprising a bottom wall part facing and forming the outer end part of the combustion chamber, a reinforcement wall part extending from the bottom wall part away from the combustion chamber, and reinforcement ribs disposed within the reinforcement wall part, the wall parts and ribs forming a cooling water passage, air passages, and exhaust gas passages.
In recent years, the requirement for higher thermal efficiency and higher power output of internal-combustion engines has given rise to the necessity of elevating the maximum pressure within the cylinders of the engines. For example, the maximum pressure within a cylinder in Kawasaki-MAN two-cycle engines was of the order of 50 to 60 kfg/cm.sup.2 in the 1950s but has risen to approximately 70 kgf/cm.sup.2 in the 1960s and to approximately 90 to 110 kgf/cm.sup.2 by 1980. In the case of Kawasaki-MAN four-cycle engines, the maximum pressure has been increased from approximately 90 kgf/cm.sup.2 in 1956 to approximately 115 kgf/cm.sup.2 in the 1960s and further to almost 150 kgf/cm.sup.2 in the 1980s.
When, in view of the above necessity for increasing the maximum pressure, the conventional cylinder head of the above described structure is considered, it is seen that the thermal stress and the mechanical stress in the bottom wall part of the cylinder head increase. As will be apparent from a stress analysis set forth hereinafter, this means that, in order to prevent a rise in the thermal stress, it is necessary to keep the thickness of the bottom wall part from increasing. Furthermore, in order to prevent the mechanical stress from rising, it becomes necessary to decrease the spans between the reinforcement ribs and, at the same time, to increase the thickness of the bottom wall part.
It becomes clear from the analysis set forth hereinafter of the thermal and mechanical stresses that sufficient strength of the cylinder head to withstand elevated maximum pressures within the cylinder without incurring an increase in the two kinds of stresses can be attained by decreasing the spans of the reinforcement ribs without increasing the thickness of the bottom wall part.
However, in a conventional cylinder head of integral cast structure, there is a limit, due to difficulties in fabrication, to the reduction of the spans of the reinforcement ribs. For this reason it has not been heretofore feasible to increase amply the maximum pressure within engine cylinders.